1. Field of Invention
The present invention pertains to a modified tumor necrosis factor-alpha converting enzyme (TACE). The present invention further pertains to generating a crystal of the modified TACE protein in protein-ligand complex with a selected inhibitor for use in structure based rational drug design. In addition, the present invention pertains to methods of using these proteins and crystals to identify compounds that can modulate the enzymatic activity of TACE.
2. Background
Tumor necrosis factor-alpha (TNF-alpha) plays a major role in the immune and inflammatory responses in mammals [Qi and Pekala, Proc Soc Exp Biol Med. 223(2):128–135 (2000)]. First isolated from the serum of rabbits that had been treated with endotoxin, TNF-alpha was named for its ability to trigger the hemorrhagic necrosis of specific transplantable tumors [Old, Science 230:630-633 (1985)]. Subsequently, TNF-alpha was found to be identical to cachetin, a protein that is intimately involved in cachexia, the wasting disease prevalent in AIDS and cancer [Beutler et al., J. Exp. Med., 161:984-995 (1985)].
TNF-alpha can bind to two distinct cell membrane receptors (TNFR1 and TNFR2) to transduce intercellular signals to a variety of different target cells in a number of different tissues [Qi and Pekala, Proc Soc Exp Biol Med. 223(2):128-135 (2000)]. Though a central participant in several key cellular processes, TNF-alpha also functions deleteriously as a mediator of insulin resistance in diabetes mellitus [Hotamisligil and Spiegelman, Diabetes 43:1271-1278 (1994)]. In addition, an over-abundance of the active form of TNF-alpha has been linked to adverse symptoms in a number of disease states, including in rheumatoid arthritis, Crohn's disease, sepsis, and cachexia. Since there are presently no effective treatments for these conditions, there remains a need to find new drugs that can be used to modulate the effective physiological concentration of the active form of TNF-alpha.
TNF-alpha is transcribed as a transmembrane protein having a monomeric molecular weight of 26 kilodaltons [Shirai et al., Nature 313:803-806 (1985)]. A metalloprotease, tumor necrosis factor-alpha converting enzyme (TACE) cleaves the membrane-associated form of TNF-alpha at a specific site of the protein, converting it to its corresponding soluble form [Black et al., Nature 385:729-733 (1997); Moss et al., Nature 385:733-736 (1997); U.S. Pat. No. 5,830,742, Issued Nov. 3, 1998; U.S. Pat. No. 6,013,466, Issued Jan. 11, 2000, the contents of which are hereby incorporated by reference in their entireties]. The soluble form of TNF-alpha then associates as a homotrimer of three 17 kilodalton monomers [Kriegler et al., Cell, 53:45-53 (1988)]. Although the membrane-associated form of TNF-alpha appears to be active, it is the proteolyzed soluble form that is responsible for the mortality associated with endotoxic shock [Gearing et al., Nature 370:555-558 (1994)]. Thus, reducing the circulating concentration of active TNF-alpha appears to be critical to alleviate the harmful side effects caused by this cytokine. One means for achieving this reduction in concentration of soluble TNF-alpha is to inhibit the TACE protease.
TACE, also referred to as ADAM 17 and CD156q, is a zinc endopeptidase that is a member of the “A Disintegrin And Metalloprotease” (ADAM) family of metalloproteases [Schlondorff and Blobel, J. Cell Sci., 112:3603-3617 (1999); Black, Intern. J. Biochem. Cell Biol 34:1-5 (2002); U.S. Pat. No. 5,830,742, Issued Nov. 3, 1998, the contents of all of which are hereby incorporated by reference in their entireties]. A type I transmembrane protein, TACE comprises (i) an extracellular region having an N-terminal signal peptide, (ii) a pro domain, (iii) a zinc-dependent catalytic domain, (iv) a disintegrin domain, (v) an EGF-like domain, (vi) a crambin-like domain, (vii) a transmembrane helix and (viii) an intracellular C-terminal tail [see WO9940182, Published Aug. 12, 1999]. More recently, the EGF-like and the crambin-like domains have been grouped together and re-named as a cysteine-rich domain [Black, Intern. J. Biochem. Cell Biol 34:1-5 (2002)].
Since TACE is a protease, the portion of the enzyme that is critical for drug discovery is its catalytic domain. The catalytic domain (TCD) of TACE comprises 263 amino acid residues preceded by a furin cleavage site (residues 211-214). The pro domain comprises a cysteine that interacts with the zinc molecule at the active-site preventing proteolytic action. Therefore, this cysteine must be displaced in order to generate an active protease [Black, Intern. J. Biochem. Cell Biol 34:1-5 (2002)].
Zask et al. have prepared a compilation of inhibitors of metalloproteinases [Curr. Pharm. Des., 2:624-661 (1996), the contents of which are hereby incorporated by reference in their entireties], and more recently, Letavic et al. has disclosed several specific inhibitors of TACE [Biorgan. & Medic. Chem Lett. 12:1387-1390 (2002), the contents of which are hereby incorporated by reference in their entireties]. To date, however, none of these has proven to be useful in the treatment of conditions related to an over-abundance of soluble TNF-alpha.
Three-dimensional structures of two different TACE-inhibitor complexes have been obtained via X-ray crystallographic analyses [Letavic et al., Biorgan. & Medic. Chem Lett. 12:1387-1390 (2002); WO9940182, Published Aug. 12, 1999, U.S. application Ser. No. 09/117,476, filed Jan. 27, 1999, the contents of which are all hereby incorporated by reference in their entireties]. Importantly, however, the conditions for preparing the two TACE-inhibitor complexes were significantly different. These results suggest that new crystallization conditions may be required for every different TACE-ligand complex. Determining crystallization conditions, de novo can be extremely time-consuming. Moreover, such a requirement severely hampers progress in identifying new and more potent inhibitors of TACE which are necessary for developing safe and effective drugs to ameliorate the deleterious effects due to an overabundance of the soluble form of TNF-alpha.
Therefore, there is need to provide methods for performing X-ray crystallographic structural determinations on multiple TACE protein-ligand complexes without having to determine crystallization conditions, de novo. In addition, there is a need to obtain X-ray diffractable crystals of the TACE catalytic domain that are stable. Furthermore, there is a need to obtain crystals of the TACE catalytic domain that are amenable to ligand exchange.
The citation of any reference herein should not be construed as an admission that such reference is available as “prior art” to the instant application.